Entities communicating with each other over a data communications network generally do so by the exchange of data packets in accordance with a predetermined protocol. Depending on the particular protocol used, the communication service provided between the entities will generally be either connectionless or connection-oriented. A connectionless service is one in which each packet is handled in isolation from any other packet, the service having no appreciation of whether or not the packet is one of a number of packets that together form a complete message. In contrast, a connection-oriented service will establish a virtual circuit between entities that wish to communicate, in order to provide a reliable stream transport service for the packets passed between the entities, the virtual circuit being closed when the entities have finished communicating with each other; such a communication path established between entities over a virtual circuit is generally referred to as a connection whilst the communication transaction carried out from the setting up to the taking down of a connection is often referred to as a call.
It is well known to provide network monitoring equipment both for the purpose of traffic monitoring and for the purpose of fault analysis. Such monitoring equipment is mostly concerned either with analyzing individual packets or with the aggregate effect of the monitored packets as a whole (for example, for traffic estimation and network planning). U.S. patent specification No. 5,101,402 discloses a somewhat more sophisticated approach that provides for the collection of statistics on sessions conducted across connections. However, a drawback of the method disclosed in U.S. Pat. No. 5,101,402 is that it must track session protocol interactions in order to ascertain when each session terminates; as a consequence, if the passage of the relevant protocol command terminating a session is missed for any reason (such as noise or packet re-routing), an error condition will result.
It is an object of the present invention to provide a method and apparatus for monitoring calls over a network that is resilient to packet loss.